In recent years, circuits for medium-voltage variable frequency drive (VFD) applications have received attention. Several novel methods have been introduced in the past decade. For example, in a circuit comprising series-connected inverters as described in U.S. Pat. No. 5,625,545 to Hammond, the disclosure of which is incorporated herein by reference in its entirety, an inverter or power cell 110 includes a three-phase diode-bridge rectifier 112, one or more direct current (DC) capacitors 114, and an H-bridge inverter 116. The rectifier 112 converts the input 118 alternating current (AC) voltage to a substantially constant DC voltage that is supported by the capacitors 114 that are connected across the rectifier 112 output. The output stage of the inverter 110 includes an H-bridge inverter 116 includes two poles, a left pole and a right pole, each with two devices. The inverter 110 transforms the DC voltage across the DC capacitors 114 to an AC output 120 using pulse-width modulation (PWM) of the semiconductor devices in the H-bridge inverter 116.
A circuit including power cells such as 110 in FIG. 1, when connected to a load, such as a motor, can provide power from an input source to the motor when operating in the motoring mode. However, when the motor speed needs to be reduced, power from the motor needs to be absorbed by the inverter. This mode of operation, when power must be absorbed by the inventor, is referred to as the regeneration mode. The diode-bridge rectifiers 112 in each power cell do not allow power to be transferred back to the source. Hence, the power absorbed by the circuit is strictly limited by the losses in the inverter and the capacitors within each power cell and is usually in the range of about 0.2% to about 0.5% of rated power.
The disclosure contained herein describes attempts to solve one or more of the problems described above.